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Old October 20th, 2015, 01:49 PM   #1
Ken Roach
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Unusual encoder failure modes ?

Hello, PLCTalk Community, I hope you can help give me a little perspective.

I'm doing an in-depth failure mode analysis on a system that uses a VFD with an ordinary differential line driver quadrature encoder for velocity feedback.

The end customer keeps questioning the reliability of the encoder signal, essentially saying "what happens if the encoder gives you a false reading and you're actually traveling at a different speed than it indicates ?"

I'm trying to convince them that such a failure mode is not a "credible failure mode".

The VFD itself recognizes four failure modes for the encoder:

1. Open Wire
2. Phase Loss
3. Quadrature Loss
4. Peripheral Module Fault

Open-wire detection depends on the differential wiring for the encoder; you have both A and A' signals for Channel A (and likewise for B and Z). If the signals are not opposite one another, you know you have a broken wire.

Phase Loss is a count of Open Wire faults; if more than 30 of them accumulate over 8 milliseconds, the drive declares an encoder phase loss.

Quadrature Loss is when the signal changes on Channel A and Channel B at the same time. Because they are offset by 90 degrees, they should never change simultaneously and if they are doing so you must be seeing a noisy signal.

Peripheral Module fault is a failure of the encoder input module or the serial bus between the drive and the module. That's made up of watchdogs and checksums.


I have been telling them that it's improbable for a noisy signal or other malfunction to create a pulse stream that is not detected by one of those three fault detection methods, but that doesn't satisfy their inquiry.

They want me to attempt to account for other failure modes in which an unforeseen signal failure or firmware glitch causes the encoder to count by multiples or to scale the incoming signal by some unforeseen multiplier or otherwise simply calculate the velocity incorrectly.

The encoder is a Dynapar magnetic ring encoder, so there are no programmable elements inside the encoder itself and a minimum of possible failures of the encoder disc/ring/pickup assembly.

So I guess my real question is: has anyone here ever seen a VFD encoder fail in a way where it gave an inaccurate velocity feedback without also having a bad signal detected ?
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Old October 20th, 2015, 02:38 PM   #2
Mike_RH
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Ken,

How about a mechanical issue - loose / slipping coupler to the encoder?
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Old October 20th, 2015, 02:51 PM   #3
RoTaTech
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Mike beat me to it - my first thought was slipping connection between the driving and driven shafts.

Another one which I am soon going to have to account for - wear in the pickup wheel. We have a composite wheel traveling on a steel track, eventually it will wear down the circumference, thus will eventually indicate a higher velocity than it should. Minimal - likely so - but if one want to be pedantic, it is a definite source of error.
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Old October 20th, 2015, 03:02 PM   #4
Ken Roach
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Good points; I will have to account for the possibility of the encoder slipping on the shaft.

These encoders are hollow-shaft style and are mounted on the motor shaft itself. The magnetic wheels are clamped to the shaft, so the possibility of slippage is as minimal as possible.

But I suppose it's possible for the magnetic wheel to collide with the sensor enclosure and spin at a rate slower than the shaft is spinning.

I'll see if I can account for that in the design. I consider it a very improbable failure, but the improbable is the order of the day.

If anyone else has experienced such a failure (on a hollow shaft encoder mounted to the motor itself, rather than a friction wheel elsewhere) I would love to hear about it.

And if you have other ideas, keep them coming !
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Old October 20th, 2015, 05:07 PM   #5
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Is the encoder connected to the drive and to a plc at the same time, you can try to make a trend out of encoder signals if possible. The other day I made a trend of one of my encoders and I found the problems was due to vibration.
My encoder was not mounted on the motor shaft.
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Old October 20th, 2015, 05:24 PM   #6
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This encoder is just wired to a VFD as the velocity feedback method. There is no PLC involved.

Widelto, what were the symptoms of your vibration problem ? Did the feedback channel fault, to did it display the wrong position or velocity, or something else ?
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Old October 20th, 2015, 06:08 PM   #7
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Ken basically what I saw on the trend were peaks were they should not be, and sometimes signals that instead of going from zero to one as they are supposed to, they appeared like a saw. Ken have you checked encoder cable, Is this cable running away from motor cables, Is the shield ok ?? Do you have any chance to use a scope ??
My project was a flying cutoff and pieces to be cut sometimes were shorter or bigger due to measuring problems.
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Old October 20th, 2015, 06:24 PM   #8
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Thanks for that feedback (pun intended) !

This system is still in the design phase, so I am not troubleshooting but rather trying to predict how a failure will affect the system.
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Old October 20th, 2015, 06:49 PM   #9
shawn_75
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The reliability they are asking for seems to point toward redundant encoders with a PLC added to continually perform a comparison.
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Old October 21st, 2015, 08:43 PM   #10
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The end customer keeps questioning the reliability of the encoder signal, essentially saying "what happens if the encoder gives you a false reading and you're actually traveling at a different speed than it indicates ?"

Wouldn't this be a good one to forward to the encoder manufacturer? They've probably got a good canned answer for this having been asked it many times before.
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Old October 22nd, 2015, 03:46 AM   #11
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I'm fairly certain that some drives monitor the speed feedback and compare that with the output frequency and will produce an error if there is more than a reasonable difference.

You can use a SIL/PL rated encoder but you still have to discount mechanical failure.

https://www.kuebler.com/english2/sildrehgeber.html

Nick
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Old October 22nd, 2015, 07:56 AM   #12
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Depending on the drive, it may throw an error if the encoder doesn't match what the drive is doing. More than likely, something in the performance will be off. It will be pulling high current because the drive is injecting current at the wrong times based on a bad encoder signal.
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